Too tight

OK so any tips and tricks to loosen myself up a little bit. My boyfriend and I could have sex three times a day and even on the third time that day he has to work to get it in because I’m too tight. Mind you have given birth four times also so I thought that I would be a little looser but that’s not the case. For example this morning I was soaking wet and it took him five minutes to get it in and afterwords I’m really really sore. I mean he’s a little thicker than average but not gigantic.